Pulping and papermaking properties of Tunisian Alfa stems (Stipa tenacissima)—Effects of refining process

Abstract

The objective of this work is to study characteristics of chemical pulps (soda cooking process) and of paper obtained from Alfa, also known as Stipa tenacissima. For this purpose, Tunisian Alfa stems, and both unbleached and bleached pulps were characterized by determining their chemical composition as well as their morphological and physical properties. Through a detailed comparison with the other pulps obtained from various species, we show that the properties of Alfa stem fibres are intermediate between those of non-wood and wood plants, and most often close to those of Eucalyptus fibres. Refining process (PFI mill device) was then applied to the unbleached and bleached Alfa pulps. The modifications of the morphological properties of the fibres and the drainability and water retention values of the pulps were studied as a function of the refining degree. Here again, Alfa fibres exhibit a behaviour similar to that of Eucalyptus fibres, as the fibre shortening is very limited during the refining process. Finally, conventional handsheets with a basis weight of 65 g/m² were prepared from the unrefined and refined pulps. Their characterization showed that Alfa based papers present low density values and quite good mechanical properties, which are significantly enhanced by the refining treatment, particularly for the unbleached pulp. This study demonstrates the high potentiality of this non-wood species for papermaking applications.

Introduction

The increasing consumption of fibre-based products relies mainly on the forest exploitation for the production of wood pulps as well as on the recycling of fibres. In European countries, for instance, more than 50% of the lignocellulosic fibres used are secondary fibres. Nevertheless, and although the use of non-wood fibres remains relatively limited in the world (around 10%), annual or perennial plants and agricultural residues constitute the major local source of cellulosic fibres in certain countries or regions like North-Africa, where forestry resources are not available or are limited. Increasing the use of these non-wood fibres by rationalizing their valorization could allow preserving the forestry resources, limiting the dependence of such countries in terms of pulps and fibres and bringing new properties to fibre-based products considering the specific properties of non-wood fibres.

Alfa, also known as Stipa tenacissima, is a common grass of North Africa, which covers approximately a surface of 3 million hectares in Algeria (Akchiche and Messaoud-Boureghda, 2007) and more than 400 thousand hectares in Tunisia, mainly located in Kasserine, Sidi Bouzid, Gafsa and Kairouan region. Alfa is considered as one of the most interesting annual plants for the production of fibres for papermaking and it has already been used in such applications for four decades (Nadji et al., 2006). For instance, in Tunisia, the production of Alfa pulp exceeds 30,000 tons per year (Nadji et al., 2006). However, and to the best of our knowledge, very few data about the properties of pulps and papers prepared from this raw material are available in the literature. In particular, no exhaustive study provides a detailed description of the effects of refining on the properties of Alfa pulps and papers. Only Bouiri and Amrani (2010), who evaluated the effect of the bleaching process on the properties of Algerian Alfa based papers, studied partially the impact of refining. Nevertheless, few mechanical properties are reported and discussed (breaking length, tear and burst indexes). That is the reason why we have decided to undertake a systematic work dealing with the effects of refining on the properties of pulps and papers obtained from Tunisian Alfa. For this purpose, the chemical composition of Tunisian Alfa stems and that of the obtained soda pulps were assessed according to standard methods. Then, the impact of refining on the morphology of the Alfa fibres (unbleached and bleached) and on their swelling behaviour was studied. Finally, the physical properties of conventional handsheets with a basis weight of 65 g/cm² prepared from pulps refined at different levels were determined. All these results are discussed and compared in the ensuing sections with data available in the literature for non-wood and wood plants.